Objectives: Use of immunofluorescence on frozen section material to look for novel or unusual proteins synthesized by human tumors. Major findings: Dedifferentiated rhabdomyosarcoma cells produce a new type of myosin immunologically related to myosin present in fibroblasts. Proposed course: Test a variety of tumors by immunofluorescence using antibodies prepared against purified proteins that have been isolated from fetal tissue, normal organs, or from other sources.